1. Field of the Invention
The present invention relates to an in-wheel motor system for use in a vehicle having direct drive wheels as drive wheels.
2. Description of the Prior Art
It is generally known that, in a vehicle having a suspension mechanism such as a spring around a wheel, as the mass of parts under the spring such as a wheel, knuckle and suspension arm, so-called “unsprung mass” increases, changes in the ground holding force of a tire when running on an uneven road become larger, thereby deteriorating road holding properties.
In a vehicle driven by a motor such as an electric car, an in-wheel motor system incorporating a motor in a wheel is being employed. However, in a conventional in-wheel motor whose non-rotating part is fixed to a spindle shaft connected to a part such as an upright or knuckle which is one of the parts around a wheel of the vehicle and whose rotor as a rotating part can rotate together with the wheel, the above unsprung mass increases by the weight of the in-wheel motor, whereby changes in the ground holding force of the tire become large, thereby deteriorating road holding properties (refer to patent documents 1 to 3, for example).
To solve the above problem, there is proposed an in-wheel motor system as shown in FIG. 6 in which a non-rotating side case 3a supporting a stator 3S is elastically supported to a knuckle 5 as a part around the wheel of a vehicle by a buffer mechanism 50 having two plates 54 and 55 whose moving directions are limited to the vertical direction of the vehicle by direct-acting guides 51 and which are interconnected by springs 52 and a damper 53 moving in the vertical direction of the vehicle and a rotating side case 3b supporting a rotor 3R and a wheel 2 are interconnected by a flexible coupling 60 as a drive force transmission mechanism which can become eccentric in the radical direction of the wheel 2 (refer to patent document 4, for example).
More specifically, as shown in FIG. 9, the above flexible coupling 60 comprises a plurality of hollow disk-like plates 61A to 61C and direct-acting guides 62A and 62B for interconnecting between the adjacent plates 61A and 61B and between the adjacent plates 61B and 61C and guiding the above adjacent plates 61A and 61B and the adjacent plates 61B and 61C in the radial direction of the disk. As shown in FIG. 10, each of the above direct-acting guides 62A and 62B comprises a guide rail 62x having a projection extending in the radial direction of the above plates 61A to 61C, a guide member 62y having a recess extending in the radial direction of the above plates 61A to 61C to be engaged with the above guide rail 62x, and a plurality of steel balls 62m interposed between the projection of the above guide rail 62x and the recess of the guide member 62y to smoothly slide the above guide rail 62x and the guide member 62y. 
Since the above guide rail 62x and the guide member 62y slide so as to guide the above adjacent plates 61A and 61B and the adjacent plates 61B and 61C in the radial direction of the disk, the in-wheel motor 3 can move in the working direction of the above direct-acting guides 62A and 62B, that is, the radial direction of the disk but not in the rotation direction. Therefore, by connecting the rotating side case 3b of the motor 3 to the wheel 2 by the above flexible coupling 60, drive torque can be transmitted from the motor 3 to the wheel 2 efficiently.
In the in-wheel motor system constituted as described above, the in-wheel motor 3 is float mounted to a part around the wheel of the vehicle by the buffer mechanism 50 so that the motor 3 itself can be used as the weight of a dynamic damper, thereby making it possible to improve ground holding performance and riding comfort when running on a bad road. Since the motor shaft and the wheel shaft are interconnected by the above flexible coupling 60 in such a manner that they can become eccentric to each other in any direction, torque can be transmitted from the motor 3 to the wheel 2 efficiently.    Patent document 1: Japanese Patent No. 2676025    Patent document 2: Japanese Examined Patent Publication No. 9-506236    Patent document 3: Japanese Unexamined Patent Application No. 10-305735    Patent document 4: WO 02/083446 A1